The present invention relates in general to multiple trace oscilloscopes and in particular to an apparatus for controlling the attributes of each trace.
In an analog oscilloscope, the path of an electron beam across the screen of a cathode-ray tube appears as a "trace" of glowing phosphor on the screen. The "attributes" of a trace relate to its appearance on the screen including its shape and intensity, and determine signals which control the vertical and horizontal deflection of the eletron beam as well as its intensity. In a multiple trace oscilloscope a single electron beam creates multiple traces on a cathode-ray tube screen, each trace having different attribute combinations. For example when traces represent different oscilloscope input waveforms, the vertical deflection of the beam is responsive to different vertical input signals for each trace. The horizontal sweep of the beam may also be accomplished by different horizontal sweep signals for each trace so that different waveforms can be simultaneously displayed using different horizontal time scales. Different signals may furthermore be used to control the intensity of the beam for each trace so that some traces appear brighter than others
In a typical multiple trace oscilloscope, the traces are periodically updated (refreshed) utilizing either an "alternating" mode or a "chop" mode of trace update. In the alternating mode of operation, the oscilloscope updates one trace during each sweep of the beam across the screen so that several sweeps are required to update all of the traces once. In the chop mode of operation, the oscilloscope updates traces during each sweep by alternately updating sections of different traces as the beam sweeps once over the screen. Therefore when the chop mode is employed, each trace is actually "chopped," having horizontal gaps occurring during the times that the beam is updating other traces. Usually, however, the alternation in trace update occurs rapidly so that the gaps in each trace are small enough so each trace has a solid appearance.
The oscilloscope selects attributes for each trace utilizing attribute control switches for selecting the appropriate signals to be transmitted to vertical, horizontal and "Z axis" amplifiers for controlling the vertical and horizontal deflection and the intensity of the electron beam. Prior to updating each trace, the oscilloscope adjusts the selections made by these attribute control switches to set the desired attributes for the trace. Typically the attributes for each trace are predetermined by an operator utilizing knobs or pushbuttons on the front panel of the oscilloscope which send signals indicating the attributes the operator wants for each trace to a state machine. The state machine has a number of output states, each corresponding to a different permissible trace attribute combination. Each time the oscilloscope switches from updating one trace to updating a next trace, its sends a clock signal to the state machine causing the state machine to enter a next state determined according to the nature of the input signals from the front panel knobs or push buttons. The state machine output comprises a set of data bits which control the attribute control switches, causing them to select the appropriate vertical, horizontal and intensity control signals for the next trace to be updated.
The state machine must have one state corresponding to each permissible combination of trace attributes. When a multiple trace oscilloscope provides a great many different vertical, horizontal and intensity signal sources and is capable of displaying many traces simultaneously, such a trace attribute control state machine would require a prohibitively large number of states in order to permit any trace to have any possible combination of attributes. Since such a large state machine is impractical, multiple trace oscilloscopes permit only limited combinations of trace attributes for each trace. In addition, since the "next state" that a state machine enters is determined by the current state that it is in and by the input data provided during the current state, such state machines typically limit the order in which states can be entered and therefore limit the combinations of traces which can be displayed at the same time. It may be possible to display a first trace having a particular combination of attributes and it may be possible to display a second trace having another particular combination of attributes, but due to the sequencing limitations of the state machine, it may not be possible to display the two traces at the same time. What is needed and would be beneficial is a trace attribute controller for a multiple trace oscilloscope which would permit any possible combination of attributes to be independently specified for each trace.